The invention relates to hydrotreating and aromatics saturation of lube oil stocks.
Lube base oils are normally manufactured by making narrow cuts of vacuum gas oils from a crude vacuum tower. The cut points are set to control the final viscosity and flash point of the lube base oil. The vacuum gas oils (or waxy distillates) are refined by either the traditional solvent refining process or a high pressure hydrocracker to remove aromatics and improve the viscosity index (V.I.). The refined waxy distillates are either hydrotreated to improve color and solvent dewaxed to reduce the pour point or catalytically dewaxed in a hydroprocessor containing both dewaxing and hydrotreating catalysts.
Lube base oils are traditionally catalytically dewaxed by a zeolyst catalyst that selectively cracks straight chain paraffins (which tend to have high pour points) to light hydrocarbons such as propane. More recently, catalysts have been developed that, in addition to their selective cracking function, isomerize (or selectively dewax) the straight chain paraffins to branched paraffins which, due to their low pour point and high V.I., are ideal lube base oils. Isomerizing instead of cracking the waxy molecules not only produces a greater yield of lube base oils but also produces base oils with a higher V.I.
These newer catalysts contain noble metals such as platinum. They were developed for use on hydrocracked feed stocks which contain very low levels of nitrogen and sulfur. Even small quantities of H.sub.2 S, NH.sub.3, organic sulfur or organic nitrogen can poison these catalysts. It is possible to use these catalysts to dewax solvent refined waxy distillates, but the distillates need to be hydrotreated first to remove sulfur and nitrogen.
In all commercial refinery processes which combine this initial hydrotreating cleanup step with a subsequent hydroprocessing step utilizing a sensitive noble metal catalyst, the process is designed, constructed and operated as essentially two separate units. As a result these processes, an example of which is catalytic reforming of naphtha for production of high octane gasoline, are expensive to construct. Typically, the initial hydrotreating section has a recycle compressor with auxiliaries and charge heater separate from the subsequent noble metal based process. In addition, the hydrotreated material from the initial step is lowered to near atmospheric pressure for stripping to remove H.sub.2 S and NH.sub.3 prior to being pumped back up to the second stage hydroprocessing pressure. This requires duplicate high pressure pumps and a low pressure stripper tower and auxiliaries.
An integrated process which avoids duplication of equipment would be very desirable.